A modular multilevel converter (MMC) is a transformerless power converter suitable for use in high- or medium-voltage power conversion applications. MMCs include a cascade connection of multiple switching submodules each having a power-storing capacitor that floats in the cascade connection. Thus, in the control of an MMC, it is necessary to individually voltage-balance and control floating capacitors in the switching submodules. The switching submodules can be constructed using various configurations of switching power transistors, such as half bridge and full bridge topologies, among others. To control the output of an MMC, the switching power transistors can be controlled using switching control signals generated by an MMC controller.
A significant amount of research has been conducted to mitigate various problems with MMC control, including the reduction of voltage ripple on the capacitors in the switching submodules. Some methods are relatively unsuitable for high voltage, high power applications, however, for various reasons. For example, some methods are unsuitable because they cause relatively large arm currents.